In the plating of dielectric substrates by chemical (electroless) plating it is well known that suitable catalytic treatment is a prerequisite for effective electroless metal deposition. Such practices are well known and accepted in the art.
In examining the prior art for catalytic treatment it appears that while different procedures have been used, the incorporation of precious metals (e.g. palladium containing solutions) was common to all procedures. One catalytic system of particular interest is the two step process as disclosed in U.S. Pat. No. 3,011,920. In the process disclosed, a colloidal solution composed of tin(II) and precious metal salts, generally with hydrochloric acid, is used. The effective catalyst is proposed to be a colloid of an elemental precious metal (e.g. palladium) stabilized by the excess stannous chloride present in the media. While the system disclosed in U.S. Pat. No. 3,011,920 has been quite popular in commercial practices, rising costs of precious metals and miscellaneous product reliability problems has led to the quest for new systems in which the use of precious metals, tin, as well as of the hydrochloric acid would be completely eliminated.
In meeting this objective it was found, as described in U.S. Ser. No. 512,224, filed by the applicant herein, that colloidal systems based upon non-precious metals could constitute the basis for new commercial plating processes. More specifically, it was found that colloidal hydrous-oxides of non-precious metals (preferably selected from the group of copper, cobalt and nickel) may be used in the direct replacement of the tin/palladium colloid followed by a treatment in a suitable reducing media. In the reducing media, reduction of the ionic portion of adduct derived through the adsorption from the colloidal media takes place, resulting in nucleating sites capable of initiation of the electroless process.
In reviewing the teaching disclosed in U.S. Ser. No. 512,224, it is recognized that many of the inherent disadvantages associated with the palladium based catalysts are eliminated. It is further recognized that based upon practices in this art that it is further essential that any catalytic system should maintain its properties especially with storage (e.g. several months) and shipment under conditions of substantial temperature fluctuations. It is thus highly desirable to have a media in which good colloidal stability would be maintained, and which at the same time has sufficient catalytic activity to be used in the plating process. I have observed that as one increases stability, activity is decreased thereby making it difficult to meet both requirements in a single system.
For example, I have observed that successful synthesis of active plating colloids has generally shown a limited stability (for long term storage purposes) due to coagulation which takes place leading to precipitation, with, of course, change in particle size and distribution during the coagulation process. In addition, I have noted that highly stabile colloidal dispersions have shown limited catalytic activity when used in accordance with Ser. No. 512,224 and moderate concentration of reducing media or activating media.
It is thus an object of this invention to provide a stable colloidal dispersion and at the same time providing a simple way by which the stable colloids may be transformed into an active catalytic form useful in electroless plating processes.